Thursday, 29 December 2016

PSS and SSS in LTE (Primary and secondary synchronization signals)

In most of websites I found where do PSS and SSS come and what do these are beneficial for. The thing which most of the LTE learners want to understand is that, "What is exactly those signals?". That is the reason I started this post.

PSS & SSS are two synchronization symbols in LTE which help UE (User equipment) to synchronize with network.

PSS is nothing but a sequence known as Zadoff-Chu Sequence. It comes in last symbol of slot 1 and slot 0. For detail info on where & how does it come, go to PSS and SSS synchronization.

Now, What is Zadoff-chu Sequence ?
In LTE, Zadoff chu sequence is a combination of complex sequences. These sequences have a special property, known as CAZAC webform. CAZAC stands for "Constant Amplitude Zero Autocorrelation". It means when we use this as a synchronization code, the correlation between the ideal sequence present in UE and a received sequence comes from eNB is greatestwhen the lag is zero. When there is any lag between the two sequences, the correlation is zero.
Here is the diagrammatic illustration:

Now, what are those 62 numbers? How can we get it?
Here is a small video of 1 min on Zadoff chu sequence used in LTE. You can see the equation shown in this video to get the answer of above question.

So, let's summarize it, UE has few sequences stored. When it is switched on, it starts scanning the network i.e it does correlation of the received signal with its own sequences. If UE gets a pick amplitude, it assumes that this is the PSS and starts waiting for the next PSS, and if it gets the pick again, it confirms that this is the PSS.
The pick denotes the middle subcarrier of the complete symbol. so it reads 31 subcarriers up and 31 subcarrier down in frequency domain to read complete PSS. That's the reason, PSS & SSS always comes in middle 72 subcarrier (62 for signal and 5 symbol is reserved each side). Once it confirmed that it listened to PSS, it starts listening to SSS. Here comes the bonus, SSS is not a Zadoff-chu sequence. Check my next post for the details about SSS.